I would like to drive Sennheiser HD 580 headphone from the speaker terminals
of a 120 watt amp. The headphones have a nominal impedance of 300 ohms,
power handling of .2 watts and sensitivity is 97dB (at 1 mW?). My intention
is to wire resistors in series across the outputs of the amp to limit the
voltage to the headphones thereby assuring that the headphones will not be
over driven and the volume control will operate in a reasonable range.

The output voltage of the amp should be about 31 volts (120 W into 8 ohms).
If I wire 4 10 ohm, 10 watt resistors in series for a load of 40 ohms across
the speaker terminals, the total output power of the amp would be about 24
watts into 40 ohms and should be low enough for the 4 10 watt resistors (40
watts) to dissipate the power and would provide about 6 volts across each
resistor. Connecting the headphones across one of the resistors would
provide a maximum of about 6 volts to drive the headphones. This
configuration would result in a maximum power of about 120 mW (6 volts into
300 ohms) which should be plenty of power without the likelihood of
destroying the headphones. The reason I chose 4 10 ohm resistors is that
this value and power rating is easy to find and cheap (Radio Shack). I
could have used 4 2 ohm resistors to provide an 8 ohm load for the amp but
the power rating of each resistor would have to be at least 30 watts
(preferably 40 or 50) which would probably be expensive and require heat
sinks. I'm assuming that the amp will perform as well into 40 ohms (or any
load as long as the power output is adequate - how about 1 watt into 1
kohm?) as it will into 8 ohms (?).

I know this project is trivial to most of the people on this group but I
would just like some confirmation that my plan is reasonable.

Thanks
John